1. Field of the Invention
The present invention relates to a method and apparatus for determining the integrity limit for a device that is connected to a Global Positioning System (GPS) receiver (sometimes referred to as the "Autonomous" receiver) and integrates the information received from the GPS receiver with another independent motion or position determining sensor for example, the gyroscopes and accelerometers in an Inertial Reference Unit (IRU), or an Attitude and Heading Reference Unit (AHRU) etc.! to determine a "hybrid" position.
2. Description of the Prior Art
Global Positioning System (GPS) is a satellite navigation system known in the art. Within airborne equipment that determines a GPS navigation position, there exists an integrity monitoring subsystem which determines the integrity limit signal associated with the calculated GPS position. The integrity limit is defined more specifically as the distance from the calculated GPS position which contains the actual position with the following constraints: 1) a 99.9% probability that the actual position is within the integrity limit, 2) does not include flight technical errors, and 3) includes all equipment failures within the entire system.
A device called a GPS receiver is used to receive signals emitted from the navigation satellites. With the information received from the satellites, the GPS receiver calculates its position along with time, velocity, and status information. This data is called the GPS "autonomous" or navigation data. Also within the GPS receiver, there exists an integrity monitoring subsystem which determines the integrity limit signals associated with the autonomous position. The GPS receiver also transmits measurement information about each satellite that it is tracking. This information includes pseudo range from the satellite, satellite position, time, range data, and health status of the satellite.
The GPS receiver determines the position and velocity of the aircraft, but it does not compute aircraft attitude. An Inertial Reference Unit (IRU) employing gyroscopes and accelerometers determines aircraft attitude and motion information. The IRU can also provide velocity and position information based off the motion detected by the gyroscopes and accelerometers, but over a period of time they tend to drift and provide less than highly accurate indications. Honeywell Inc. produces a Global Positioning Inertial Reference Unit (GPIRU) identified as an HG1050AG01 which is referred to as a "hybrid" system because it provides outputs which are a combination of the GPS and IRU information. More particularly, the GPIRU includes an inertial reference subsystem with gyros and accelerometers to provide information about aircraft attitude and rate of change of position from which an inertial position can be determined. The GPIRU receives the autonomous data, autonomous integrity limits and satellite measurement information from the GPS receiver. It uses the satellite measurement information as a second and independent source for determining aircraft position and velocity. The inertial reference data is integrated with the GPS satellite measurement information in the GPIRU to produce a set of hybrid navigation signals. Like the GPS receiver, the GP1RU must also compute an integrity limit signal associated with the computed hybrid position. In the prior art, an integrity monitor subsystem is included in the GPIRU to perform this task.
The integrity monitoring subsystem is a costly, complex, time and space consuming apparatus. By eliminating the integrity monitoring subsystem in either the GPS receiver or the GPIRU reduces the cost and complexity associated with developing the applicable product. This invention eliminates the need of having an integrity monitoring subsystem within the GPIRU by calculating the hybrid integrity limit based on the autonomous integrity limit, the autonomous position and the hybrid position.
It should be understood that while the GPIRU is used as an example of a device that calculates a hybrid position by integrating the GPS receiver's data with the inertial reference data, other devices are also capable of producing a hybrid position. The present invention is applicable to not only the GPIRU, but, as stated above, to any device that meets the criteria of being connected to the GPS receiver and integrating the information received therefrom with another independent source of attitude or position information.